Methods of lining pipes with a rigid plastic tube have already been proposed wherein the plastic tube is inserted through the pipe and thereafter inflated radially of the pipe by applying heat and pressure to the tube from inside to cover the inner surface of the pipe with the tube. The lining formed by the proposed method is made of the inflated rigid plastic tube and therefore has shape retentivity, high strength and a high quality.
Although the pipe lining work differs with the type of pipe, the work is performed for a length of pipe which is usually about 10 to about 100 m as single span. The rigid plastic tube serving as the lining material has a considerably large length longer than one span of pipe, so that the tube can be transported to the work site conveniently when it is wound on a drum. The rigid plastic tube can be wound on the drum when collapsed radially thereof from the form having a circular cross section to a flat form and made flexible by being softened with heating.
To pay out the plastic tube from the drum, there arises a need to heat and soften the tube at the work site as when it was wound on the drum. However, since the plastic tube wound on the drum is in the form of layers of a flat tube, it is difficult to directly heat the layers. Further because plastic materials generally have low heat conductivity, the layers of tube can not be heated effectively through heat conduction. Accordingly, even if the tube winding on the drum is heated, the outermost layer may be softened but the underlying layers remain unsoftened, with the result that the plastic tube can not be smoothly paid out from the drum and inserted into the contemplated pipe and thereby entails a low work efficiency.
When inserted into the pipe, the plastic tube gradually releases heat, becomes rigid again and substantially loses flexibility, so that another problem arises in that the tube can not be passed through an intermediate bend or like portion of the pipe.